Field of the Invention
The present invention relates to a pneumatic tire where main grooves extending along the tire circumferential direction are formed in a tread portion.
Background Art
When a vehicle travels on pebbles road or the like, there may arise so-called pebble biting which is a phenomenon that pebbles on a road is bitten in a main groove formed in a tread potion of a tire. When the vehicle continues traveling in such a state where pebble biting occurs and the tire rolls, the pebble bitten in the main groove presses a groove bottom or a force which intends to widen the main groove by pushing acts on the groove bottom and hence, the damage such as cracks is liable to occur in the groove bottom.
In view of the above, conventionally, a pneumatic tire where projections are formed on a groove bottom or groove side walls of a main groove thus preventing pebble biting by such projections has been proposed in JP-A-2007-210568, JP-A-2002-370510 and JP-A-4-183609.
Although the above-mentioned tire can suppress the occurrence of pebble biting in the vicinity of the groove bottom, the tire has a drawback that the biting of pebbles in the vicinity of an opening end of the main groove cannot be prevented. It may be possible to increase a projecting height of the projections from the groove bottom. In this case, however, the rigidity of the projection is lowered. Accordingly, there arises a drawback that the biting of pebbles into the groove cannot be prevented or a volume of the groove is decreased and hence, traveling performance of the tire on a wet road surface (wet performance) is lowered.
Further, in the above-mentioned tire, the projection formed in the main groove is present on the groove bottom and hence, the projection has a shape where the projection is raised steeply from the groove bottom and hence, a gentle curved shape cannot be imparted to a boundary portion between the groove bottom and the projection. Accordingly, there arises a drawback that a crack is liable to occur from a root portion of the projection (the boundary portion between the groove bottom and the projection) which constitutes a crack starting point.